Coastal Erosion on Sydney’s southern beaches Study Report at the Water Research Laboratory of the University of New South Wales, Sydney, Australia in association with the Bergische Universität Wuppertal Univ.-Prof. Dr.-Ing. H. Kaldenhoff by cand.-Ing. Marcus Dätig Sydney, January 2001 Contents 1 Introduction 1 2 Beaches in the coastal zone 2 3 The Beach System 5 4 Coastal erosion and beach processes 6 5 Human Response to Natural Coastal Hazards 9 5.1 The Establishment of Buffer Zones . 9 5.2 Beach Protection Systems . 9 5.2.1 Seawalls . 9 5.2.2 Groynes . 10 5.3 Sand Nourishment . 10 5.4 Dune Stabilization . 10 6 Three Southern Beaches of Sydney 13 6.1 Clovelly Beach . 14 6.2 Coogee Beach . 15 6.3 Maroubra Beach . 16 7 The Coastline Hazards of these beaches 20 7.1 Clovelly Bay . 20 7.1.1 Zone 1: hazard level: minor to moderate . 20 7.1.2 Zone 2: hazard level: minor to serious . 20 7.1.3 Zone 3: hazard level: minor . 21 7.1.4 Zone 4: hazard level: minor to serious . 22 7.1.5 Zone 5: hazard level: low . 22 i CONTENTS ii 7.2 Coogee Bay . 23 7.2.1 Zone 1: hazard level: minor . 23 7.2.2 Zone 2: hazard level: serious . 24 7.2.3 Zone 3: hazard level: minor . 25 7.2.4 Zone 4: hazard level: moderate to serious . 25 7.3 Maroubra Bay . 26 7.3.1 Zone 1: hazard level: moderate . 26 7.3.2 Zone 2: hazard level: serious . 27 7.3.3 Zone 3: hazard level: moderate to serious . 28 7.3.4 Zone 4: hazard level: minor to moderate . 28 7.3.5 Zone 5: hazard level: minor to moderate . 29 7.4 Final Statement . 29 Bibliography 31 List of Figures 2.1 Coastal Environment and processes . 2 2.2 Sediment gain and Sediment loss . 3 3.1 The Beach system . 5 4.1 Generation and Propagation of a wave . 6 4.2 Swell and Storm Profile . 7 5.1 Description of a Buffer Zone . 12 6.1 Location of the southern beaches . 13 6.2 Coogee beach subdivided in different areas . 16 6.3 Maroubra Beach, part of the ’Plan of Management’ . 19 7.1 Zones of the Clovelly Bay . 21 7.2 Clovelly Bay . 22 7.3 Clovelly Beach . 23 7.4 Zones of the Coogee Bay . 24 7.5 Coogee Beach, view from the south . 25 7.6 Coogee Bay, view from the north . 26 7.7 Zones of the Maroubra Beach . 27 7.8 Maroubra Beach . 28 7.9 Maroubra coast . 29 iii 1 Introduction This report about the 3 southern beaches of Sydney starts by giving a brief introduction about the coast of New South Wales. The state of NSW is located in the southeast of Australia and is one of the oldest and most populated states in Australia. Consisting of an area of 801.506 square kilometres, which can be regarded as only 10 % of the whole continent, approximately 30% of the Australian inhabitants live there. The New South Wales coast extends from Tweed Heads in the north to Cape Howe in the south, a distance of 1590 km. Sixty percent of the coast, or 975 km consists of 721 sandy beaches. The other 615 km can mostly be described as rocky coast. The beaches range from long sweeping beaches on the north coast, up to 26 km in length, to small often deeply embayed beaches, many only a few meters in lengths. The coastal region of Sydney north for example with a length of 45 km has got 20 sandy beaches with an average length of 625m, compared to the coastal region south of Sydney, being 46 km long, but showing just an average length of 359m within 19 sandy beaches (1). Coastal Protection and coastal management being neglected in the early years of our century, by the 1960s it was becoming evident that coastal erosion and its effects are a serious problem. Private homes, commercial buildings, public facilities and even roads were all endangered. The New South Wales government initiated an investigation in which a study had been made about the coastal processes regarding the coastal recession. This study revealed average rates of coastal recession of 0.5 to 1.0 meter a year, an amount of the coast to be lost in the ocean, which is quite threatening. This study has been one of several reasons why the New South Wales government introduced the Beach Improvement Programme in 1975 to assist local authorities in restoring and improving the recreational amenity of the state’s beaches. This program was based upon technical work carried out by the Public Work Department (nowadays changed to the Department of Land and Water Conservation) and was used also as a means for increasing public awareness of coastal problems (2). 1 2 Beaches in the coastal zone The coastal zone includes the underlying geology, the coastal hinterland, the inner continental shelf and the coastline including all coastal bays and beaches. The coastal environment is the most dynamic part of the earth surfaces. It contains elements of all spheres that make up earth namely the atmosphere, the hydrosphere, or ocean, the lithosphere, or geology and the biosphere. As the 4 spheres interact, (see figure 2.1) at the coast they produce a wide spectrum of coastal systems ranging from muddy tidal flats to rocky headlands to sandy beaches (2). Figure 2.1: Coastal Environment and processes (1) The atmosphere contributes the regional climate, the winds, rain and temperature and indirectly the ocean temperature. The hydrosphere determines sea level and due to this also the position of the shoreline, as well as contributing the major energy sources waves, tides and currents. Finally the biosphere includes the flora and fauna that inhabit either permanently, or in transit the coastal zone and contribute shell, coral and algal fragments that help to build the beaches. As this report will describe three of the sandy beaches of the southern beaches of Sydney, it first of all will show some facts about sandy beaches in general. 2 2 Beaches in the coastal zone 3 They consist of lithospheric elements namely sand lying ultimately on bedrock geology which is permanently been influenced by the interacting spheres mentioned above. Beaches form and change due to several processes that interact at the coast to produce beach systems. Two of those are to be regarded as climate and atmospheric processes. Climate interacting with the geology together with the physical forces of the rain let the part of the land surface runoff as well as rivers erode and transport sediment to the coast (2). Figure 2.2: Sediment gain and Sediment loss (9) At the coast it is also climate, particularly winds that interact with the ocean to generate waves and currents that are as already mentioned essential to build this sediment into beaches and dunes. Particularly on the coast of New South Wales the beaches are predominantly composed of quartz sand grains together with other rock fragments. These sediments originated in the coastal hin- terland. Over many millennia they have been delivered by rivers and streams, to the coast and continental shelf, and subsequently moved by waves on- or along-shore to be finally deposited as beaches and dunes (1). The regional geology therefore acts as a source for most of the sand on NSW beaches, the remainder being derived from shell and algal fragments. The rocks of the NSW coast that form the headlands and cliffs range in age from 500 million to 60 million years. As the coast is only 60 million years old, most of the rocks predate the coastline by many million years (1). Age of NSW coastal features: • Rocks: from 500 million years old • Beaches: from 12.000 years old • Most beaches: less then 6.500 years old The rock of the cliffs in the area that is going to be described in this report is known as the Hawkesbury Sandstone. It was laid down as sediment in a large river delta, which stretched 2 Beaches in the coastal zone 4 from north of the Hawkesbury River south almost to Nowra and west to lower Blue Mountains. This Sandstone was forming about 200 million years ago. The rock is made up mainly of sand and quartz in layers which vary according to what else the rivers brought down at different times and the way they were flowing. There is also an occasional band of shale, composed of more muddy material. Unlike the rock itself, the cliffs are very young regarding the geological time dimensions. During the last ice age the shoreline was about 12 km east, across the coastal plain. As the ice melted around the globe, sea level rose across the plain to plateaux of sandstone along the present shoreline. By the time it stopped rising about 6.000 years ago, the sea had flooded into a series of river valleys cut into this sandstone, creating bays and harbours along the new coast (1). 3 The Beach System Different types of beaches show different types of profiles, depending on the influence of the littoral zone. The figure below illustrates a typical NSW beach system Figure 3.1: The Beach system (2) On the left hand side of figure 3.1 you can see the subaerial beach which is a dry zone above the shoreline, only influenced by the sea through storm waves, or when there are high tides being exceptionally high. The sediment on the subaerial beach becomes dry quickly and creates a series of coastal sand dunes with the help of wind, presupposing the coastline is nearly flat.